The Paradox of Epistemic Obligation (Åqvist 1967)

(1) The bank is being robbed.
(2) It ought to be the case that Jones (the guard) knows that the
bank is being robbed.
(3) It ought to be the case that the bank is being robbed.

Let us symbolize “Jones knows that the bank is being
robbed” by “Kjr.” Then
it would appear that a correct way to symbolize (1) - (3) in SDLs
(augmented with a “K” operator) is:

(1′) r
(2′) OBKjr
(3′) OBr

But it is a logical truth that if one knows that p then
p is the case (surely Jones knows that the bank is being
robbed only if the bank is in fact being robbed).
So ⊢
Kjr → r would hold in any
system augmented with a faithful logic of knowledge. So in such a
system, it would follow by RM
that ⊢
OBKjr →
OBr, but then we can derive (3′) from
this conditional and (2′) by
MP.[1]
But it hardly seems to follow from the fact that it is obligatory
that the guard knows that the bank is being robbed, that it is
likewise obligatory that the bank is being robbed. It seems that SDL
countenances inferences from patently impermissible states of affairs
that someone is obliged to know hold when they hold to the conclusion
that the same impermissible states of affairs are
obligatory.[2]